lkml.org 
[lkml]   [2017]   [Nov]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [question] extract the feature bits width to 4
From
Date

On 2017/11/28 19:40, Suzuki K Poulose wrote:
> Cc: linux-arm-kernel
>
> On 28/11/17 11:17, gengdongjiu wrote:
>> Hi,suzuki/mark,
>
> Hello!
>
> Please Cc linux-arm-kernel mailing list in the future for any arm/arm64 kernel
> related queries.
Thanks a lot for the reply, Ok

>
>>
>>    very sorry to disturb you, I have a question that want to consult with you. For the CPU feature detection,
>> why we use extract 4 bits width for the feature match instead of the actual bits number[1]?  may be the actual hardware feature bit more than 4 bits.
>
>
> There are features which could be more or less than 4bit wide. However,
> all the arm64_cpu_capabilities features are 4bits and hence the hard coded
> bits there. If we ever come across handling capabilities which are not
> standard size, we could add the necessary code to support it.
yes, normally the arm64_cpu_capabilities features are 4bits. However, in my
platform, there is a private CPU feature which is more than 4bits.

>
> Are you planning to add something which does require the change ? Please
> feel free to send patches.

If I support this feature, I have to change it and not use 4bits. This CPU
feature is private, I does not want to upstream the whole feature.

If I only send the change about using actual width instead of 4 bits when detection feature ,
I am not sure you are agree.


>
> Cheers
>
> Suzuki
>
>
>> thanks!
>>
>> static inline int __attribute_const__
>> cpuid_feature_extract_field(u64 features, int field, bool sign)
>> {
>>          return cpuid_feature_extract_field_width(features, field, 4, sign);
>> }
>>
>> static bool
>> feature_matches(u64 reg, const struct arm64_cpu_capabilities *entry)
>> {
>>          int val = cpuid_feature_extract_field(reg, entry->field_pos, entry->sign);
>>
>>          return val >= entry->min_field_value;
>> }
>>
>>
>
>
> .
>

\
 
 \ /
  Last update: 2017-11-28 13:40    [W:0.066 / U:4.568 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site